The duke and duchess of Cambridge, Prince William and Kate Middleton, announced last month that they were expecting. The couple has two children, 4-year-old Prince George and 2-year-old Princess Charlotte.

Kensington Palace officials said in a statement that Queen Elizabeth and other family members were "delighted with the news."

The duchess, 35, suffered from hyperemesis gravidarum, or severe morning sickness, during her first two pregnancies, and Kensington Palace officials said last month that she was again dealing with the illness through her third pregnancy.